Many of the phenomena concerned with the passage of metabolites across natural membranes are incompletely understood. The interpretation of biological transport experiments usually rests on relating in vivo data to conceptual models of transport mechanisms. We are devising experimental methods to examine individual transport mechanisms in detail especially those associated with microporous membranes and hindered diffusion. By finding the quantitative relationships between transport rates through chemically and physically defined membranes and the properties of the membranes and permeants, we will provide an experimental and theoretical foundation for the understanding of biological transport phenomena. This information will be valuable in interpreting fluid balance changes, capillary exchange, drug distribution, kidney ailments, and in tailoring synthetic membranes for specific purposes such as blood oxygenation and hemodialysis.